This invention relates to a flexible coupling which is of annular shape and comprises a plurality of fixing portions spaced circumferentially about the coupling and leaves extending between each adjacent pair of fixing portions. The latter are arranged for connection to two rotary members which are to be coupled. In use one rotary member is connected to alternate ones of the fixing portions and the other rotary member is connected to the other fixing portions.
The term xe2x80x9cannular shapexe2x80x9d is used herein to include not only a circular ring but also a polygon or other shape having an aperture surrounded by the leaves. The polygon may be made up of a number of separate links each comprising a leaf extending between fixing portions at the end of the leaf. The invention also includes such links.
Couplings of the type described above are known made as a unitary composite (fibre-reinforced plastic) laminated structure. Each leaf has a comparatively small thickness in the direction axially of the coupling, i.e. parallel to its axis of rotation in use, and has a relatively large width in the direction radially of the coupling (i.e. radially of such axis).
When torque is applied to a flexible composite coupling of the type described in the preceding paragraph, tensile strain develops in those leaves subjected to tensile forces and a compressive displacement is imposed on those leaves subjected to compressive forces. Small compressive displacements may be accommodated without buckling by compressive strain in the leaf but typical compressive displacements cause buckling of the leaves. In a flexible composite coupling rotating at 0xc2x0 installed angle, i.e. with the axes of the coupled rotary members aligned, the leaves retain their buckled shape throughout continuous operation whilst transmitting torque. When a flexible coupling is used at an installed angle greater than zero, i.e. where there is angular misalignment between the axes of rotation of the coupled rotary members, tensile strain is imposed on all the leaves. If the strain due to the installed angle is smaller than the strain due to applied torque, stable buckling of the leaves loaded in compression is retained throughout rotation. This corresponds to the area of the operating diagram shown in FIG. 1 above the shaded zone. If the strain due to the installed angle is bigger than the strain due to applied torque, then there is no buckling of the leaves loaded in compression. This corresponds to the area of FIG. 1 below the shaded zone. A bi-stable buckling phenomenon is observed in the transition zone of the operating diagram because a condition occurs once per revolution where a displacement sufficient to cause buckling is imposed on the leaves loaded in compression. Symmetrical leaves are free to accommodate this displacement by buckling either way and they oscillate once per revolution between the alternate shapes caused by the bi-stable buckling motion.
Such a bi-stable buckling motion, i.e. the change of displacement of the central part of each leaf from being displaced axially in one direction from an unstressed position to being displaced axially in the other direction from said position, is noisy and can damage the coupling.
It is an object of the invention to provide a coupling which avoids this problem.
According to the invention we provide flexible annular coupling made of composite (fibre-reinforced plastic) material and comprising an even number of relatively thicker fixing portions spaced apart circumferentially around the coupling for connection to two rotary members to be coupled by the coupling and a flexible leafextending lengthwise between each two adjacent fixing portions, each leaf comprising a relatively thinner central part and transition portions of gradually increasing thickness between the ends of the central part and the relatively thicker fixing portions; characterized in that the coupling is so constructed that the central part of at least each circumferentially alternate leaf will always be displaced axially (as defined above) in the same direction transverse to its length when such leaf is caused to buckle by compressive force(s) acting between the fixing portions at the ends of the leaf
It is only necessary, in theory, that alternate ones of the leaves are arranged so that they buckle in a pre-determined direction in compression if these can be arranged to be the leaves which will be in compression in use. However if a coupling is to be used to transmit torque in both senses, (i.e. positive and negative torques) then the central portion of each leaf must buckle in a pre-determined direction when the leaf is longitudinally compressed. It is not necessary that the central parts of all the leaves be displaced in the same direction on buckling, it is only necessary that, for a particular leaf, its central part will always be displaced in the pre-determined direction when the leaf buckles under compression.
Various constructions may be used to ensure that the central parts of the leaves are displaced in a pre-determined direction on compressive buckling of the leaves. Thus the transition portions at the ends of each leaf may be shaped so as to effect the buckling in the pre-determined direction. Alternatively, or in addition, the central portion of each leaf which is required to be displaced in a pre-determined direction on buckling may be made of two layers of material one of which is stiffer than the other to resist lengthwise compression. Each layer may extend over the whole width of the central parts of the leaves. The central part of the leaf will then displace towards the stiffer layer on buckling. The required difference in stiffness may be effected by selection of specific fibre orientations relative to the axis of the leaf and/or by the use of reinforcing fibres of different moduli.
In another and preferred arrangement the central part of each of those leaves in which the central part is to be displaced in a pre-determined direction on buckling is, in an unstressed state, curved between its ends so that the curved part projects axially in the pre-determined direction in which the central part will be displaced when the leaf buckles under compression. The central part projects to one side, and one side only, of a plane perpendicular to the rotary axis of the coupling and passing through the axial mid-points of fixing elements.
The radius of curvature of the central part in this case may be between 10 and 500 times the axial thickness of the central part of the leaf. The central part of each curved leaf may have a central, offset curved portion between intermediate portions of uniform thickness which are parallel to said plane and extend from the central portion to the adjacent ends of the transition portions. The length of each intermediate portion may be between L/50 and L/10 where xe2x80x9cLxe2x80x9d is the distance parallel to said plane between the centres of fixing holes in adjacent fixing portions. The thickness (S) of the central portion and of the intermediate portions may be between L/250 and L/25 where L is as defined above.
The thickness (T) of each fixing portion may be between 2S and 10S where (S) is as defined above. Each transition portion may extend a distance of between 0.1 and 0.4L from the centre of a fixing hole in its adjacent fixing portion where L is as defined above.
The maximum offset of the central part of the leaf measured between the neutral axis of the curved part and said plane may be between L/250 and L/25 where L is as defined above.
The radial width of the central part of each leaf may be at least 10 times its axial thickness. There have been many proposals to make flexible couplings of a plurality of thin metal rings or links. Examples are U.S. Pat. Nos. 1,947,052; 3,759,064 and 4,768,992. The metal rings or links include curved portions between their ends. In some cases the curves are sinusoidal and extent on both sides of the neutral axis of the ring or link.
In all cases the curved portions are said to be included to improve the flexibility of the coupling, i.e. to enable the coupling to transmit torque when the axes of the driving and driven members connected by the coupling are angularly misaligned.
The maximum angular misalignment at which a laminated steel coupling can operate at its rated torque is typically about 0.5xc2x0, said couplings being restricted to the allowable working strain of steel. A single lamina metal diaphragm coupling would have an even lower maximum angle of misalignment of about 0.2xc2x0 at which it could operate at its rated torque.
It has also been proposed in DE-A-41 40 311 to have a flexible annular coupling made of fibre-reinforced plastic material comprising an even number of fixing portions spaced apart circumferentially around the coupling for connection to two rotating members. A flexible leaf extends lengthwise between each two adjacent fixing portions. The coupling includes apertured reinforcing collars associated with the fixing portions. The collars and reinforcing portions have complementary interfitting formations which maybe conical or cylindrical.
A fibre composite coupling of the type with which the invention is concerned can typically operate at its rated torque when the driving and driven shafts have an angular misalignment of about 2xc2x0 with a corresponding higher working strain than is acceptable for steel. It is the working strain which provides the energy which causes the bistable buckling referred to above. The lower working strain in a steel coupling would not normally produce the phenomenon. A fibre composite coupling may operate for a restricted time at an angular misalignment up to 4xc2x0.
Not surprisingly, therefore, none of the patents dealing with metal flexible couplings deals with how to avoid bistable buckling. There is no suggestion in the patents that to curve the leaves in a fibre composite coupling would provide a solution to bistable buckling. The curves in the steel links or rings are provided as a means to increase flexibility.
There is, in fact, a contra-indication against curving the leaves of a fibre composite coupling. Through-thickness stresses in a composite coupling tend to delaminate the coupling since it does not have much strength in the through-thickness direction. Curving the leaves produces through-thickness stresses in the tension leaves as they tend to straighten when the coupling is transmitting torque. So this is another reason why a skilled man would not have considered curving the leaves to be a solution to the bi-stable buckling problem with which he is faced.
The aim of the present invention is to cause the compression links to buckle in a pre-determined direction. All the different ways of effecting the buckling of the leaves in a pre-determined direction have, as a common feature, that the central parts of the leaves, and/or the transitional portions if provided are asymmetrical either in shape or construction with respect to a plane perpendicular to the rotary axis of the coupling and passing through the axial mid points of adjacent fixing portions. This is described in more detail below.
The coupling may be made as a unitary member of composite material or as a number of separate links of composite material each of which comprises two fixing portions with a leaf extending between them, the fixing portions of adjacent links overlapping where the links are secured to a rotary member to be coupled. The construction makes for ease of servicing.